Transformer windings are usually made of copper magnet wire, but aluminum is a suitable competitor where weight and first-time cost are decisive factors.

Transformers are devices that transfer electrical energy from one circuit to another through its coils (windings). The characteristics required by motor windings are similar to those required by transformers, but they also need to withstand mechanical vibration and centrifugal force at operating temperature.

In North America, aluminum is the main choice for winding materials for low-voltage dry-type transformers greater than 15 kiloamperes (kVA). In most other parts of the world, copper is the main winding material. The purchase decision is usually a function of the loss assessment, expressed in currency per kilowatt.

The copper used in the manufacture of transformer windings is the wire form of small products and the wire form of large equipment. For small products, the wire must be strong enough to be wound without breaking, but flexible enough to provide close-packed windings. Strip products must have the good surface quality to prevent the insulating enamel from cracking under voltage. Good ductility is essential for strip forming and packaging, and it also needs good strength to withstand the high electromechanical stresses that occasionally occur under short circuit conditions. The copper winding wire in the transformer is compatible with all modern insulating materials such as paint and enamel. The lacquer allows close spacing of the windings to provide the best efficiency in the coil.

Spatial considerations are the main engineering reason for choosing copper windings instead of aluminum. This is because the copper-wound transformer can be made smaller than the aluminum transformer. In order to obtain the same rating in an aluminum transformer, its cross-sectional area needs to be 66% larger than that of a copper conductor. However, the use of a larger size conductor will cause the strength of the aluminum winding to be almost equivalent to that of the copper winding.

Connectivity is another important advantage of copper-wound transformers. There is no need to use high-quality joint compounds for cleaning to prevent round enameled wire oxidation.